SELECT 
  cscart_ab__mb_motivation_item_objects.motivation_item_id, 
  cscart_ab__mb_motivation_item_objects.object_type, 
  GROUP_CONCAT(
    cscart_ab__mb_motivation_item_objects.object_id
  ) as object_ids 
FROM 
  cscart_ab__mb_motivation_item_objects 
WHERE 
  1 
  AND cscart_ab__mb_motivation_item_objects.motivation_item_id IN (1, 2, 3) 
  AND cscart_ab__mb_motivation_item_objects.object_type IN ('C', 'D') 
GROUP BY 
  cscart_ab__mb_motivation_item_objects.motivation_item_id, 
  cscart_ab__mb_motivation_item_objects.object_type

Query time 0.00041

JSON explain

{
  "query_block": {
    "select_id": 1,
    "read_sorted_file": {
      "filesort": {
        "sort_key": "cscart_ab__mb_motivation_item_objects.motivation_item_id, cscart_ab__mb_motivation_item_objects.object_type",
        "table": {
          "table_name": "cscart_ab__mb_motivation_item_objects",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "mid_otype"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["motivation_item_id"],
          "rows": 6,
          "filtered": 100,
          "attached_condition": "cscart_ab__mb_motivation_item_objects.motivation_item_id in (1,2,3) and cscart_ab__mb_motivation_item_objects.object_type in ('C','D')",
          "using_index": true
        }
      }
    }
  }
}